Quantum dots (QDs) comprise colloidal semiconductor cores that are small, often spherical, crystalline particles composed of group II-VI, III-V, IV-VI, or I-III-VI semiconductor materials. Each semiconductor core is a nanocrystal consisting of hundreds to thousands of atoms. Quantum dots are neither atomic nor bulk semiconductors, but may best be described as artificial atoms. Their properties originate from their physical size, which ranges from about 1 to about 10 nanometers (nm) in radius, and are often comparable to or smaller than the bulk Bohr exciton radius. As a consequence, quantum dots no longer exhibit the optical or electronic properties of their bulk parent semiconductor. Instead, they exhibit novel electronic properties due to what are commonly referred to as quantum confinement effects. These effects originate from the spatial confinement of intrinsic carriers (electrons and holes) to the physical dimensions of the material rather than to bulk length scales. One of the better-known confinement effects is the increase in semiconductor band gap energy with decreasing particle size; this manifests itself as a size dependent blue shift of the band edge absorption and luminescence emission with decreasing particle size.
As the nanocrystals increase in size past the exciton Bohr radius, they become electronically and optically bulk-like. Therefore they cannot be made to have a smaller band gap than exhibited by the bulk materials of the same composition, implying that the longest wavelength that can be emitted by a quantum dot is equivalent to the bulk band gap energy. Thus, quantum dots comprise materials with band gaps less than 0.413 eV and 0.248 eV for 3 micron and 5 micron emission respectively.
The band gap and the resulting absorption onset and emission wavelength are determined by the nanocrystal size. Each individual nanocrystal emits light with a line width comparable to that of atomic transitions. Any macroscopic collection of nanocrystals, however, emits a line that is inhomogeneously broadened due to the fact that every collection of nanocrystals is unavoidably characterized by a distribution of sizes. Presently the highest quality samples can be produced with size distributions exhibiting roughly a minimum of 5% variation in nanocrystal volume. This directly dictates the width of the inhomogeneously-broadened line which corresponds to ˜35 nm for CdSe, ˜70 nm for InGaP, and ˜100 nm for PbS. These same material systems can be tuned to have a peak emission wavelength from 490 nm “blue” through the visible and the short wavelength infrared to 2300 nm.
The absorption spectra are dominated by a series of overlapping peaks with increasing absorption at shorter wavelengths. Each peak corresponds to an excitonic energy level, where the first exciton peak (i.e. the lowest energy state) is synonymous with the blue shifted band edge. Short wavelength light that is absorbed by the quantum dot will be down converted and reemitted at a shorter wavelength. The efficiency at which this down conversion process occurs is denoted by the quantum yield. Non-radiative exciton recombination reduces quantum yield due to the presence of interband states resulting from dangling bonds at the quantum dot surface and intrinsic defects. Quantum yields can be greatly increased to nearly 90% in some circumstances by passivating the surface of the quantum dot core through the addition of a wide band gap semiconductor shell to the outside of the nanocrystal.
The nanocrystals or semiconductor cores are typically coated with one or more inorganic semiconductor shells, each of which is typically 0.1-10 monolayers thick, or about 1 angstrom to 2 nm thick. Common shell compositions include, but are not limited to, wide band gap semiconductors such as zinc sulfide and cadmium sulfide. The shells serve to increase the quantum yield (brightness) of the photoluminescent emission by occupying surface dangling bonds and defects that tend to cause non-radiative interband states.
Quantum dots are usually enveloped by a layer of surfactant molecules having one or more functional groups that bind to the metal atoms on the quantum dots surface (examples of the functional groups include, but are not limited to, phosphine, phosphine oxide, thiol, amine carboxylic acid, etc.) and one or more moieties on the opposite end from the metal-binding groups to increase the solubility of the quantum dot in a given solvent or matrix material. For example, hydrophobic aliphatic, alkane, alicyclic, and aromatic groups on the distal ends of the surfactant molecules increase the solubility of the quantum dots in hydrophobic solvents, while polar or ionizable groups increase the solubility of the quantum dots in hydrophilic and aqueous solvents.
Quantum dots are sensitive to the chemistry of the environment in which they reside. Defects such as dislocations, atomic vacancies, or oxide bonds can be introduced onto quantum dot surfaces in acidic or oxidative conditions or in the presence of radicals, certain catalysts, and other reactive compounds. Defect formation is exacerbated when the quantum dots are illuminated. The prevalence of defect is related to the density of interband states and hence the probability of non-radiative recombination events. The overall result is that in certain chemically reactive and photoxidative environments the quantum yields of the quantum dots are greatly and irreversibly diminished. However, many applications of quantum dots require that they reside in these environments.
Furthermore, sulfur atoms, which are one component of zinc sulfide shells that are frequently used to passivate nanocrystal cores, as well as amine moieties, which is often a component of the surfactant layer that envelopes the nanocrystal cores, may adversely affect the matrix material in which the quantum dots are dispersed. For example, both sulfur and amines effectively reduce the activity of platinum-based catalysts that are frequently used to crosslink two-part silicones. These silicones are frequently used as encapsulant materials for LEDs, solar cells, and other optoelectronic devices.
To date, microparticles containing quantum dots have been developed by dispersing quantum dots in a liquid phase polymeric matrix materials (examples include various plastics, silicones, and epoxies), curing or drying the composite into a solid form, and then milling the composite into micron scale particles. However, these particles suffer drawbacks. Organic matrix materials degrade under intense illumination and under high energy (i.e. short wavelengths such as ultraviolet) light. Further, many organic materials have relatively low melting points or may degrade at elevated temperatures. Many organic polymers, particularly silicones, are also very permeable to oxygen, which may attack the quantum dots dispersed therein.
Methods of dispersing or coating quantum dots in an inorganic matrix such as silica have been shown in the art. For example, others have used tetraethylorthosilicate (TEOS) to glass-coat nanocrystals. However this and similar approaches greatly diminish the nanocrystals' quantum yield.